The main objective for the operational stage of an oil reservoir, from a technical-economic standpoint, is to obtain optimal recovery of hydrocarbons, i.e., maximize the economic value of the reservoir, so that the residual oil saturation is the smallest possible. To reduce this amount of the left over oil in the reservoir, it is used secondary and/or improved recovery processes, which consist primarily of fluid injection to provide additional energy and/or a favorable change of some properties of the rock-fluid system. The benefit would be a better displacement of oil towards producing wells, thereby increasing the recovery factor of the reservoir.
One of the most adverse factors to any fluid injection project is the presence of heterogeneities. The failure to timely detect them and therefore, not considering their influence on the project, can significantly reduce the likelihood of success of the same, and even lead to failure. Application of tracer tests between oil wells has recently gained prominence in the oil industry since this type of tracer tests is a good technique to investigate the behavior of the injection fluid flow in reservoirs and to determine the properties of the rock-fluid system that controls the gas and water displacement processes. The tracers have been used in many projects of secondary and tertiary recovery as a technique to quantify sweeping efficiencies and heterogeneities of the reservoir.
Tracer tests have been used to reduce the uncertainty attributed to communication between wells, horizontal and vertical flow and residual oil saturation. Based on a thorough review of the technical literature, it may be noted that the analysis of tracer tests has been mostly qualitative. As reported in the literature, it can be concluded that a poor sampling due to inadequate design is one of the main factors that leads that it does not obtain the expected results from the tracer tests. Also, it can be concluded that quantitative analysis of this type of tests is very limited, either analytical or numerical, and very few are reported with advanced numerical modeling. According to Y. Du and L. Guan, 2005, tracer tests between oil wells, most of them (61%) in a qualitative way, from the remainder (39%): 14% were analyzed by numerical methods and 25% with analytical analysis.
Several methods have been proposed to monitor the injected fluids for recovery of hydrocarbons, for example, U.S. Pat. No. 5,168,927 which discloses a method that provides a strong advance for tracers by injecting a relatively small amount volume of tracer at large pace, using a flow induced by production wells to transport the tracer; measurements of residual oil and sweeping can be obtained from this method. Another example is U.S. Pat. No. 4,099,565 which presents a method to obtain data useful in assessing the effectiveness or to design an improved recovery process by determining the hydrocarbon saturation in the formation. There is also U.S. Pat. No. 3,933,131 in which the oil flow path is monitored through the injection of a stable radical, or by spin level, within the reservoir as a tracer which becomes detectable in a sample taken from the producing well. Also, U.S. Pat. No. 4,273,187 in which a method is presented for determining the amount of recovery of petroleum chemicals retained within a reservoir through the collection of data from at least one injection-soaking-production cycle in a single well, the produced fluids are monitored through the chemical concentration of the produced fluid. Simulated cycles are repeated until the concentration of the chemical of simulated fluid produced is virtually the same concentration in the actual fluid produced. The amount of chemicals is then calculated by conventional techniques. Another method related to the present invention is U.S. Pat. No. 4,482,806 which discloses a method of registering a plurality of formations where first and second gamma radioactive tracers of different energy levels are introduced into the formation. The records are produced by tracers as traced fluids when passing through the formation and records are analyzed to determine changes in effective permeability and the sweeping of formations.
Recently, other methods have been presented relating to the present invention. For example, U.S. Pat. No. 7,472,748 proposes a method for determining more approximated properties of the formation and/or compression of the fracture, through fluid identity data for a plurality of return fluid samples; and using a reservoir model, with the fluid identity data and one or more properties of the reservoir. Another method is proposed in U.S. patent application US 2009/0211754 A1 in which a fluid can be tracked in a well using at least one WID label (wireless identification), such as a LW label (long wave length identification), entrained in the fluid. WID tag reader may be disposed and/or moved in the well, for example, a drill string or a casing string. A reader can be used to locate at least one WID tag in the well. A reader can be placed in the drill string (sub). A fluid entrained with at least one WID can be used as a tracer fluid.
In U.S. patent application US 2010/0006292 A1, methods and systems are described to stimulate oil wells. A method considers contacting the formation with a treatment fluid and monitoring the movement of the treatment fluid in the reservoir providing one or more sensors for measuring the temperature and the pressure, which is placed on a support adapted to maintain a given spacing between the sensor and the exit fluid. In some realizations, the support pipe is flexible.
Also mentioned is U.S. Pat. No. 5,072,387 which presents a method for determining the transit time of a radioactive tracer for determining the steam injection profiles. The radioactive decay data are collected in two detectors at different depths. Then the data is transformed to a new set of data comprising the time intervals between decay events. The arrival time of the tracer is determined as the first time in which a minimum detectable radiation is specified.
Additionally, it is also provided a method for characterizing reservoirs in U.S. Pat. No. 5,305,209 which presents a method for characterizing multi-layer reservoir through a single layer model representative of the flow parameters of a multilayer reservoir and developing a set of flow rate predictions from a numerical simulator. Differences between actual and simulated flow rates are automatically minimized to obtain the flow parameters for each layer of the multilayer reservoir.
However, given the experience gained to date in inter-well tracer tests, it is noted that the analysis is difficult because there are no complete design methods which integrate elements such as analytical and numerical modeling that allow predictions and based on these achievements to get a better design. Nor is there any method that integrates all stages of a tracer test (design, operation and interpretation). In the absence of these key elements in the design stage, it is likely that tracer test will not produce the expected results for some of the following relevant points: i) poor selection of injector well, ii) Inadequate tracer, both type and quantity, iii) poor selection of the wells monitored, either in number and in areas, iv) poor sampling program, among others. These unsubstantiated designs of tests lead to scarce tracer responses and is not possible to obtain useful information from them. As a consequence, it is impossible to obtain response curves from the tracer which may be interpreted or possible to perform a quantitative analysis thereof. Incorporation of a tracer activity measurement system on line gives new elements which allow successful testing of tracers. These elements are, for example, a continuous measurement of tracer passing through the production line, that is, the absence of data is completely eliminated; human errors are avoided in the time to time sampling, problems caused by climate or by bad weather are also eliminated, as well as not having data in critical test times, etc.
Also, it is noted that analytical modeling may be difficult because representative models of tracers flowing through porous media are not known. At this point, it is also important to mention that a significant percentage of reservoirs worldwide (geothermal and hydrocarbons) are found in naturally fractured formations, and most of the available modeling tracer tests in porous media are not applicable to this type of reservoir, due to the high heterogeneity of the same and all the processes that can occur when the tracer moves through fractured porous media; macroscopic processes, such as convection and dispersion, and microscopic such as diffusion, chemical reaction, ion exchange, adsorption and radioactive decay, which may be present and must be considered in the analysis.
Quantitative analysis of tracer tests depends on the ability to properly describe all processes that influence tracer travel throughout the reservoir.
Similarly, applicants do mention that one of the main problems that arises in interpreting the results of a tracer test are the result of poor and/or insufficient monitoring program. This, according to reports from Du, 2005, primarily is due to inadequate design. Also, it can be attributed to inadequate operation (one or more of the design parameters are not satisfied). This may be from an inappropriate injection, the amount of tracer injected is not verified, samples are not collected in accordance with the program, whether for climate type issues or other simpler issues, and these changes are not considered in the interpretation of the test.
Therefore, one object of the present invention is to provide to special elements necessary to enable integral analysis of tracer tests, considered from the test design up to its interpretation, leading to the determination of properties of the reservoir (including connectivity between wells, existence of barriers and/or conductive faults, etc.) and the global behavior of injection fluids, as well as improvement of the numerical model of the field in the zone involved in the test.
Further, another object of the present invention is to provide a method intended to meet the requirement of having an Integral analysis method of inter-well tracer tests which considers each of the relevant aspects mentioned above, which is based on the dynamic interaction between the modules of design, operation and interpretation, as well as the work lines conforming such modules.
Thus, through the use of the present invention, valuable information can be obtained from this type of testing, so that its consideration in the fluid injection processes tends to increase secondary or tertiary production of hydrocarbons.
The application of the method presented here allows the user an integral analysis of tracer tests, both qualitatively and quantitatively, as are additional elements that impact a highly supported, systematic and integral analysis.